As one of optical transmission techniques, discrete multi-tone (DMT) modulation, which is a type of orthogonal frequency-division multiplexing (OFDM), is known (see Patent Document 1, for example). In this technique, by configuring, for each subcarrier, the number of bits to be transmitted by using a corresponding subcarrier, in accordance with a signal-to-noise ratio (may also referred to as an S/N ratio or SNR) of an electrical signal corresponding to an optical signal received by an optical receiver, high-speed optical transmission is realized in accordance with transmission characteristics.
In addition, there is a need for applying the DMT modulation to wavelength-division multiplexing (WDM), because the DMT modulation has high efficiency of frequency utilization and high cost advantage. By applying the DMT modulation to the WDM, high-speed optical communication is realized.
Incidentally, regarding a light source including a laser diode, since a wavelength of light emitted by the light source is highly dependent on temperature, the temperature of the light source is controlled to be constant regardless of variation of optical output power and the like emitted from the light source, by using a temperature controlling element such as a thermoelectric cooler (TEC).
However, in the WDM, multiple signals having different wavelengths are densely multiplexed. Accordingly, if a wavelength of light emitted from a light source deviates from a wavelength as a reference (hereinafter referred to as a “reference wavelength”), a signal to be transmitted may be overlapped with another signal having adjacent wavelength, and a signal characteristic may degrade.
The following is reference document:    [Patent Document 1] United States Patent Application Publication No. 2014/0099115